Soil Moisture Neutron Probe Data (FIFE)

Summary:

The neutron-probe data present a series of measurements of volumetric water content in the soil profile compiled using the neutron method. These data were collected from throughout the FIFE study area from May 1987 through August 1989.

The neutron method of measuring soil water content uses the principle of neutron thermalization. When both hydrogen and oxygen are considered, water has a marked effect on slowing or thermalizing neutrons. Thermal neutron density is easily measured with a detector, if the capture cross-section remains constant then the thermal neutron density may be calibrated against water concentration on a volume basis.

1. Data Set Overview:

Data Set Identification:

Data Set Introduction:

The neutron-probe data present a series of measurements of volumetric water content in the soil profile. These data were collected throughout the FIFE study area from May 1987 through August 1989.

Objective/Purpose:

The focus of this data collection was to measure profiles of soil-moisture content at intervals of 20 cm to a depth of 2 meters or to the level of impenetrable rock layers, whichever was shallower.

Summary of Parameters:

Soil moisture at several depths.

Discussion:

The neutron-probe data present a series of measurements of volumetric water content (volume of water in the sample/volume of total sample) in the soil profile. These measurements are not directly comparable with the gravimetric water content measured at the soil surface (weight of water in the sample/weight of dried sample). Because the sphere of influence of the neutron-probe instrument would extend into the air above the soil surface, measurements in the top 20 cm of the soil were determined gravimetrically and converted to volumetric values using measurements of bulk density. These data were collected throughout the FIFE study area from May 1987 through August 1989. During this time range measurements were made only during the growing seasons of 1987 and 1988 and during late summer (July and August) of 1989.

Requested Form of Acknowledgment.

The Neutron Probe Soil Moisture data were collected for FIFE by the staff and students of the Evapotranspiration Laboratory at Kansas State University under the direction of Dr. Edward Kanemasu. The dedicated effort of A. Nelson, T. Shah and G. Harbers in the collection and preparation of these data is particularly appreciated.

3. Theory of Measurements:

The neutron method of measuring soil water content uses the principle of neutron thermalization. Hydrogen nuclei have a marked property for scattering and slowing neutrons. High-energy neutrons (5.05 [MeV]) emitted from a radioactive substance such as radium-beryllium or americium-beryllium slow down and change direction by elastic collisions with atomic nuclei (thermalization). The energy of the neutrons are reduced to about the thermal energy of atoms in a substance at room temperature. Considering both energy transfer and scattering cross-section, it is evident that hydrogen, having a nucleus of about the same size and mass as the neutron, has a much greater thermalizing effect on fast neutrons than any other element. When both hydrogen and oxygen are considered, water has a marked effect on slowing or thermalizing neutrons. Thermal neutron density is easily measured with a detector, if the capture cross-section, except for that due to water, remains constant (i.e., chemical composition constant), then the thermal neutron density may be calibrated against water concentration on a volume basis.

4. Equipment:

Sensor/Instrument Description:

The neutron moisture depth probe and meter consisted of 4 items:

Fast neutrons (usually americium-241/beryllium),

A detector of thermalized neutrons,

A protective shield composed of lead (for gamma ray absorption), and

Paraffin (for neutron absorption).

The paraffin also served as a reference standard. The neutron source was a small capsule located on the side of the detector cylinder.

A soil auger was used to create the opening for the thin-walled aluminum access tubing. The diameter of the auger was slightly smaller than the tubing to assure a tight fit.

Collection Environment:

Ground.

Source/Platform:

Ground.

Source/Platform Mission Objectives:

Determination of volumetric soil moisture content.

Key Variables:

Soil moisture by volume at 14 depths.

Principles of Operation:

Thermalization of neutrons by water.

Sensor/Instrument Measurement Geometry:

The zone of influence for 5-MeV neutrons is roughly spherical with a radius of about 15 cm in wet soil and 70 cm in dry soil.

Manufacturer of Sensor/Instrument:

CPN Corporation 2830 Howe Road Martinez, California 94553.

Calibration:

In 1988, the two neutron probes used during FIFE were calibrated at site 802 (SITGRID_ID 1816) and at one other site within the Konza Prairie Natural Research Area in the northwest quadrant of the FIFE study area. Neutron counts under saturated and very dry, soil moisture conditions were taken. Immediately after taking the neutron counts, soil samples were collected to obtain the soil moisture contents.

Specifications:

50 milli-Rem per hour source of Americium/Beryllium

Tolerance:

Approximately +/- 0.005 [g][cm^-3].

Frequency of Calibration:

Once.

Other Calibration Information:

None.

5. Data Acquisition Methods:

The soil auger is used to form the hole for installation of the access tubing. The access tubing usually is left so as to protrude about 10 cm above the soil surface and is covered with an empty can or stopper between readings to keep water and debris out.

A measurement is made by placing the probe unit over the access tube preparatory to lowering it into the hole. An appropriate counting time (15 seconds during FIFE measurements) is shield. Then one or more counts are made at each selected depth. The calibration curve is used to convert the count ratios (count in soil/count into volumetric water content, or to read water content directly if the equipment has the required built-in computer.

6. Observations:

Data Notes:

Not available.

Field Notes:

There were 55 stations. Collocated stations are coded using the lowest station number. Those stations are:

Stations 11, 16 & 18 together are coded as: 11

Stations 17 and 22 together are coded as: 17

Stations 19 and 24 together are coded as: 19

Stations 21 and 26 together are coded as: 21

Stations 25, 30 & 32 together are coded as: 25

Sample location in relation to the center of the site were represented according to the following codes during 1988: 1: Central part of station location. 2: North of station location. 3: West of station location. 4: South of station location. 5: East of station location. Locations 2 through 5 are located approximately 30 m from the station.

During 1989 the 5 neutron probe access tubes were installed with the AB (Wind Aligned Blobs - see FIFE experiment plan for 1989) sector Shashi's site (stations 11, 16 and 18) was laid out differently, 8 neutron tubes (plus two additional locations which were sampled for daily soil moisture measurements within 10 cm of the surface). To emphasize the different physical layout of this station, those locations are coded as follows:

11: 150 m from center, approximately 10 degrees North of due East 12: 100 m from center, approximately 10 degrees North of due East 13: 150 m from center, approximately 10 degrees North of Southeast 14: 100 m from center, approximately 10 degrees North of Southeast 15: 150 m from center, approximately 10 degrees East of due South 16: 100 m from center, approximately 10 degrees East of due South 17: 150 m from center, approximately 10 degrees West of due South 18: 100 m from center, approximately 10 degrees West of due South 19: 100 m from center, approximately Southwest 20: 60 m from center, approximately Southwest Volumetric soil moisture measurements were made at the following depths:

Dry gravimetric samples weighing more than 65 grams were assumed to have been collected with the volumetric sampler and volumetric water content was calculated via the following equation: (Wet weight - Dry weight)/volume Smaller samples were converted to a volumetric basis via the following equation: (Percent by weight) X (bulk density) With the following exceptions, the surface gravimetric samples were collected on the same day as the neutron readings during 1987:

Station Neutron Gravimetric
------- ------- -----------
17 June 16 June 15
19 June 16 June 15
20 June 16 June 15
21 June 16 June 15
28 June 16 June 15
36 June 16 June 15
38 June 16 June 15
17 July 22 July 20
19 July 22 July 20
20 July 21 July 20
21 July 22 July 20
28 July 22 July 20
36 July 21 July 20
38 July 21 July 20
44 July 22 July 20
17 August 1 July 30
19 August 1 July 30
20 July 31 July 30
21 August 1 July 30
28 August 1 July 30
36 July 31 July 30
38 July 31 July 30
44 August 1 July 30
2 August 7 August 8
4 August 7 August 8
10 August 7 August 8
11 August 5 August 8
17 August 7 August 8
19 August 7 August 8
20 August 5 August 6
21 August 7 August 8
28 August 7 August 8
36 August 5 August 6
38 August 5 August 7
2 August 14 August 13
4 August 14 August 13
21 August 13 August 11
2 August 23 August 21
4 August 23 August 21
9 August 20 August 21
10 August 23 August 21
19 August 22 August 21
21 August 22 August 21
28 August 22 August 21
44 August 19 August 20
9 September 5 September 2
13 September 4 September 2
15 September 4 September 2
17 September 4 September 3
19 September 5 September 3
20 September 4 September 3
21 September 4 September 3
23 September 4 September 2
28 September 5 September 3
29 September 4 September 2
36 September 2 September 3
38 September 2 September 3
44 September 4 September 3
1 September 16 September 19
2 September 16 September 15
3 September 16 September 19
4 September 16 September 15
5 September 16 September 19
9 September 14 September 19
10 September 16 September 15
13 September 14 September 19
15 September 14 September 19
17 September 14 September 15
19 September 14 September 15
20 September 14 September 15
21 September 14 September 15
23 September 14 September 19
28 September 14 September 15
29 September 14 September 19
36 September 17 September 15
44 September 14 September 15
17 September 26 September 24
19 September 26 September 24
20 September 26 September 24
21 September 26 September 24
28 September 26 September 24
36 September 25 September 24
44 September 25 September 24
2 October 4 October 5
4 October 4 October 5
10 October 4 October 5
20 October 3 October 5
36 October 3 October 5
44 October 6 October 5
2 October 18 October 15
4 October 18 October 15
10 October 18 October 14
17 October 19 October 14
19 October 19 October 14
20 October 19 October 15
21 October 19 October 15
28 October 19 October 15
36 October 17 October 15
44 October 19 October 15

7. Data Description:

Spatial Characteristics:

The FIFE study area, with areal extent of 15 km by 15 km, is located south of the Tuttle Reservoir and Kansas River, and about 10 km from Manhattan, Kansas, USA. The northwest corner of the area has UTM coordinates of 4,334,000 Northing and 705,000 Easting in UTM Zone 14.

Spatial Coverage Map:

Not available.

Spatial Resolution:

These are point data.

Vertical resolution varied with depth. It ranged from 1.25 cm at in the top layer to 10 cm in the deepest layer (180-200 cm).

Horizontal resolution was 30 meters for all sites except Station 11 (sitegrid = 4439-NPK). For this site the horizontal resolution ranged from 60-150 m from the center of the site. The Field Notes Section gives a detailed listing of the sampling resolution at Station 11 (sitegrid ID = 4439).

Projection:

Not available.

Grid Description:

Not available.

Temporal Characteristics:

Temporal Coverage:

The overall time period for collection of these data was from May 28, 1987 through August 10, 1989. Soil samples were collected from late spring through the fall of 1987 (May 28 - November 6) and 1988 (April 11-September 29), and during late summer of 1989 (July 24-August 10). During these periods there were 58, 71 and 9 days of data, respectively.

Temporal Coverage Map:

Not available.

Temporal Resolution:

There were 136 days of data. During the Intensive Field Campaigns the data were collected weekly, at other times the measurement interval was 10 days to 2 weeks.

Data Characteristics:

The SQL definition for this table is found in the SM_NEUT.TDF file located on FIFE CD-ROM Volume 1.

The primary certification codes are: EXM Example or Test data (not for release) PRE Preliminary (unchecked, use at your own risk) CPI Checked by Principal Investigator (reviewed for quality) CGR Checked by a group and reconciled (data comparisons and cross checks)

The certification code modifiers are: PRE-NFP Preliminary - Not for publication, at the request of investigator. CPI-MRG PAMS data which is "merged" from two separate receiving stations to eliminate transmission errors. CPI-??? Investigator thinks data item may be questionable.

8. Data Organization:

Data Granularity:

There were 136 days of point data. Coverage for vertical measurements ranged from 2.5 cm to 200 cm with measurements made at 14 discrete depths. Vertical resolution varied with depth. It ranged from 1.25 cm at in the top layer to 10 cm in the deepest layer (180-200 cm).

Horizontal resolution was 30 meters for all sites except Station 11 (sitegrid = 4439-NPK). For this site the horizontal resolution ranged from 60-150 m from the center of the site.

A general description of data granularity as it applies to the IMS appears in the EOSDIS Glossary.

Data Format:

The CD-ROM file format consists of numerical and character fields of varying length separated by commas. The character fields are enclosed with a single apostrophe. There are no spaces between the fields. Each file begins with five header records. Header records contain the following information: Record 1 Name of this file, its table name, number of records in this file, path and name of the document that describes the data in this file, and name of principal investigator for these data. Record 2 Path and filename of the previous data set, and path and filename of the next data set. (Path and filenames for files that contain another set of data taken at the same site on the same day.) Record 3 Path and filename of the previous site, and path and filename of the next site. (Path and filenames for files of the same data set taken on the same day for the previous and next sites (sequentially numbered by SITEGRID_ID)). Record 4 Path and filename of the previous date, and path and filename of the next date. (Path and filenames for files of the same data set taken at the same site for the previous and next date.) Record 5 Column names for the data within the file, delimited by commas. Record 6 Data records begin.

Each field represents one of the attributes listed in the chart in the Data Characteristics Section and described in detail in the TDF file. These fields are in the same order as in the chart.

9. Data Manipulations:

Formulae:

Derivation Techniques and Algorithms:

An empirical equation for water content in terms of the count ratio, used over the range of water contents of usual interest, is as follows:

THETA = a + bf. where: f is the count rate ratio, I/I(std),a and b are parameters which depend upon soil characteristics and the standard count when the neutron source and detector are in the shield.

R = 100 cm / (1.4 + 10 m)

where: R = radius of the sphere of influence accounting for 95% of the neutron flux which would be obtained in an infinite medium (Olgaard, 1965).m = water content in [g][cm^-3]. Volumetric water content was calculated via the following equation: (Wet weight - Dry weight)/volume.

Data Processing Sequence:

Raw counts were logged in the field, further processing of the data, to obtain the soil volumetric moisture contents, was carried out on a personal computer in the lab.

Processing Steps:

Not available.

Processing Changes:

None.

Calculations:

For the near-surface measurements (SOIL_MOISTURE_75MM and SOIL_MOISTURE_25MM) dry gravimetric samples weighing more than 65 grams were assumed to have been collected with the volumetric sampler and volumetric water content was calculated via:

Special Corrections/Adjustments:

In the 1988 and 1989 data -9.9 indicates missing values.

Calculated Variables:

Not available.

Graphs and Plots:

None.

10. Errors:

Sources of Error:

Errors could arise from spatial variation in water content which depend on spatial variation in soil constituents, profile structure and the degree to which calibration conditions resemble particular field conditions. Such variations can involve bias as well as unidentified error associated with site variation.

When sharp interfaces or wetting-fronts exist, the shape or size of the volume of influence may be altered as the water content changes. Such changes would lead to a volume of influence that is not centered on the neutron sensor. An additional factor involves perturbations in the water content caused by temperature changes in the soil surrounding a neutron-probe access-tube induced by heat conduction along the tube. Some errors in water content measurement may also occur with random neutron emission.

Quality Assessment:

Data Validation by Source:

Not available.

Confidence Level/Accuracy Judgment:

Proper techniques using a neutron probe were followed and these data should be as good as possible using this technique.

Measurement Error for Parameters:

Not available.

Additional Quality Assessments:

FIS staff applied a general Quality Assessment (QA) procedure to the data to identify inconsistencies and problems for potential users. As a general procedure, the FIS QA consisted of examining the maximum, minimum, average, and standard deviation for each numerical field in the data table. An attempt was made to find an explanation for unexpected high or low values, values outside of the normal physical range for a variable, or standard deviations that appeared inconsistent with the mean. In some cases, histograms were examined to determine whether outliers were consistent with the shape of the data distribution.

The discrepancies which were identified are reported as problems in the Errors Section.

Data Verification by Data Center:

The data verification performed by the ORNL DAAC deals with the quality of the data format, media, and readability. The ORNL DAAC does not make an assessment of the quality of the data itself except during the course of performing other QA procedures as described below.

The FIFE data were transferred to the ORNL DAAC via CD-ROM. These CD-ROMs are distributed by the ORNL DAAC unmodified as a set or in individual volumes, as requested. In addition, the DAAC has incorporated each of the 98 FIFE tabular datasets from the CD-ROMs into its online data holdings. Incorporation of these data involved the following steps:

Copying the entire FIFE Volume 1, maintaining the directory
structure on the CD-ROM.

Using data files, documentation, and SQL code provided on
the CD-ROM to create a database in Statistical Analysis System
(SAS).

Each distinct type of data (i.e. "data set" on the CD-ROM), is accompanied by a documentation file (i.e., .doc file) and a data format/structure definition file (i.e., .tdf file). The data format files on the CD-ROM are Oracle SQL commands (e.g., "create table") that can be used to set up a relational database table structure. This file provides column/variable names, character/numeric type, length, and format, and labels/comments. These SQL commands were converted to SAS code and were used to create SAS data sets and subsequently to input data files directly from the CD-ROM into a SAS dataset. During this process, file names and directory paths were captured and metadata was extracted to the extent possible electronically. No files were found to be corrupted or unreadable during the conversion process.

Additional Quality Assurance procedures were performed as follows:

Statistical operations were performed to calculate minimum
and maximum values for all numeric fields and to create a listing
of all values of the character fields. During this process, it
was determined that various conventions were used to represent
missing values. (Note: no modifications were made to any data
by the DAAC). In most cases, missing value identification conventions
were discussed in the accompanying .doc file. Based on a visual
check of the minimum and maximum values, no glaring errors or
holes were identified that might indicate errors introduced during
CD-ROM mastering by the FIFE project or data ingest by the DAAC.

Some minor inconsistencies and typographical errors were identified
in some of the character fields and column labels, however, no
modifications were made to the data by the DAAC.

Some conversions of ASCII data were necessary to move the
data from a DOS platform to a UNIX platform. Standard operating
system conversion utilities were used (e.g., dos2unix).

Much of the metadata required for archival is imbedded in
the narrative documentation accompanying the data sets and extracted
manually by DAAC staff who have read the .doc files provided on
the CD-ROM and have hand entered this information into the metadata
database maintained by the DAAC. QA procedures have been performed
on these metadata to identify and eliminate typographical errors
and inconsistencies in naming conventions, to ensure that all
required metadata is present, and to ensure the accuracy of file
names and paths for retrieval.

Data requested for distribution to users are checked to verify
that files copied from disk to other media remain uncorrupted.

As errors are discovered in the online tabular data by investigators, users, or DAAC staff, corrections are made in cooperation with the principal investigators. These corrections are then distributed to users. CD-ROM data are corrected when re-mastering occurs for replenishment of CD-ROM stock.

11. Notes:

Limitations of the Data:

Not available.

Known Problems with the Data:

None.

Usage Guidance:

This data set could be used in conjunction with other soil moisture data to validate the soil moisture values predicted by the airborne remote sensing instruments during FIFE. It could be used with caution in similar prairie landscapes to compare remote sensing derived soil moisture and field measured soil moisture.

Any Other Relevant Information about the Study:

None.

12. Application of the Data Set:

This data set could be used in conjunction with other soil moisture data to validate the soil moisture values predicted by the airborne remote sensing instruments during FIFE.

13. Future Modifications and Plans:

The FIFE field campaigns were held in 1987 and 1989 and there are no plans for new data collection. Field work continues near the FIFE site at the Long-Term Ecological Research (LTER) Network Konza research site (i.e., LTER continues to monitor the site). The FIFE investigators are continuing to analyze and model the data from the field campaigns to produce new data products.

14. Software:

Software to access the data set is available on the all volumes of the FIFE CD-ROM set. For a detailed description of the available software see the Software Description Document.

Data Center Identification:

Procedures for Obtaining Data:

Users may place requests by telephone, electronic mail, or FAX. Data is also available via the World Wide Web at http://daac.ornl.gov.

Data Center Status/Plans:

FIFE data are available from the ORNL DAAC. Please contact the ORNL DAAC User Services Office for the most current information about these data.

16. Output Products and Availability:

The Neutron Probe Soil Moisture data are available on FIFE CD-ROM Volume 1. The CD-ROM file name is as follows:

\DATA\SOILMSTR\SM_NEUT\GRIDxxxx\Yyyyy\ydddgrid.SMN

Where xxxx is the four digit code for the location within the FIFE site grid, yyyy are the four digits of the century and year (e.g. 1987). Note: capital letters indicate fixed values that appear on the CD-ROM exactly as shown here, lower case indicates characters (values) that change for each path and file.

The format used for the filenames is: ydddgrid.sfx, where grid is the four-number code for the location within the FIFE site grid, y is the last digit of the year (e.g. 7 = 1987, and 9 = 1989), and ddd is the day of the year (e.g. 061 = sixty-first day in the year). The filename extension (.sfx), identifies the data set content for the file (see the Data Characteristics Section) and is equal to .SMN for this data set.

Archive/DBMS Usage Documentation.

The Collected Data of the First ISLSCP Field Experiment is archived at the EOS Distributed Active Archive Center (DAAC) at Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee (see the Data Center Identification Section). Documentation about using the archive and/or online access to the data at the ORNL DAAC is not available at this revision.

20. Document Information:

April 26, 1994 (citation revised on October 15, 2002).

Warning: This document has not been checked for technical or editorial accuracy by the FIFE Information Scientist. There may be inconsistencies with other documents, technical or editorial errors that were inadvertently introduced when the document was compiled, or references to preliminary data that were not included on the final CD-ROM.

Previous versions of this document have been reviewed by the Principal Investigator, the person who transmitted the data to FIS, a FIS staff member, or a FIFE scientist generally familiar with the data.